FIG. 1 shows a cross sectional side view of a conventional chip package 1 comprising an integrated circuit 11 mounted on a multi-layer printed circuit board 12. Wire bonds 13 connect pads of the integrated circuit 11 to printed circuit board wiring 14 that provide electrical connection to balls 15 at the lower surface of package 1. The integrated circuit 11 and wire bonds 13 are protected by cover 16.
As integrated circuit technology has progressed, both integrated circuits and the boards and systems into which they are installed have become more complex. Integrated circuit packages are available with a very large number of pins for connecting circuits inside the integrated circuit to the board. Indeed, ICs are known with close to 1000 pins or balls. As the number of pins increases, the difficulty of making a board with connecting lines (routing resources) to the pins also increases. Connecting lines on the board typically lead to other integrated circuit devices. It is desirable to minimize the number of these lines needed to form a desired system.
One solution to the proliferation of lines on a board is to stack several integrated circuit packages on top of each other. Behlen et al. in U.S. Pat. No. 5,598,033 describe a stacking scheme for micro-BGA packages so as to provide a high density integrated circuit package. FIG. 2, taken from FIG. 3 of Behlen et al., shows a package that can be stacked. As shown in FIG. 2, package 2 includes an interposer 312 comprising an upper layer 340 and a lower layer 338. Balls 314 at the lower surface connect to connectors below, for example to another integrated circuit package or to a printed circuit board. Contact pads 326 connect to structures above, for example another identical package. FIG. 3, taken from FIG. 4 of Behlen et al, shows a stack of such packages. Such a structure has the disadvantage of increasing the height of the system board that must be allowed for the stack.
Another solution to the problem of increasing complexity and proliferation of lines on a system printed circuit board is to form multi-chip packages or modules (MCMs). Several chips having many connections between themselves are placed into a single package. They are interconnected through a small board or large chip within the package, and the board or the chips are connected to pins that extend from the package. Thus a MCM package installed into a board requires less board area and wiring than would the individual chips within the package, and also requires fewer lines on the board to make the interconnections. The structure may also require less height than the stack of FIG. 3.
FIG. 4 is taken from FIG. 1 of Degani et al. U.S. Pat. No. 5,646,828 and shows a MCM structure 17 comprising integrated circuit chips 18, 19, and 20. These chips are located in a cavity 16 of the package 4. Chip 18 interconnects chips 19 and 20 and connects to the outside through wires 22 to contact pads 23 and then via thruholes 24 to other levels of the printed circuit board and to contacts 25 and solder bumps 26 at the lower surface of the printed circuit board. FIG. 4 also shows connections to other chips or electronic devices such as 27 and 28 on the upper surface of the printed circuit board. Further discussion of these two prior art structures appears in the respective patents and is incorporated herein by reference.
MCMs are expensive to manufacture, and a MCM requires considerable planning ahead to assure that the modules are available to install into boards when they are needed. Further, upgrading one of the chips in a MCM requires a redesign of the MCM as well as the chip within the MCM.
Thus it is desirable to have a package that is both inexpensive, easy to upgrade, and low profile, as well as a package able to minimize the need for wiring resources on a printed circuit board.
It is also desirable to prevent observation of the signals being passed between chips.